(1) Field of the Invention
The present invention relates to a curl forming method for a can end in a can end manufacturing process, and more specifically, to a curl forming method for a can end capable of providing curl formation for a thin can end without inducing wrinkles.
(2) Description of the Prior Art
In a conventional curl formation of a can end, a can end punched by a press is formed with a curl by a curl forming apparatus of a rotary rail curl system as shown in FIG. 2. This curl forming apparatus consists of a rotating curling wheel 20, a curling guide segment 21 and a curling center guide 22. A can end 25 is supported by the curling center guide 22. The distance between the curling guide segment 21 and the curling wheel 20 is gradually narrowed while the can end 25 is being sandwiched between the rotating curling wheel 20 and the curling guide segment 21 and rolled whereby the can end is gradually reduced in diameter and subjected to curl forming.
A further method so far proposed is a forming method for performing curling forming using a forming die on the basis of a press system (JPA 60-158930 publication).
Recently, to make a can end material thinner has been strongly demanded in an attempt of saving resources and reducing the cost of can ends. However, when a can end having a thin wall-thickness is subjected to curling by a conventional curl forming apparatus, there arises the following problems. The thickness of a can end material capable of maintaining a good curling work is now at a substantial limit so that it is difficult to make the can end material further thinner.
(1) A slip occurs during curing work to fail to form an outer diameter of a curl into a shape close to round. Wrinkles caused by rolling tend to occur. PA0 (2) Wrinkles are generated in the extreme end of the curl. The wrinkles are grown larger by rolling to lower the sealability of the can end. PA0 (3) The strength in the radial direction of the can end is reduced due to the thinner thickness so that when forming, the internal surface of the can end tends to be flexed, and the internal surface of the can end tends to be damaged by the center guide. PA0 (4) The irregularities in thickness of a curl of a can end increases to make it impossible to accurately manage the number thereof in a bag. PA0 (5) In the case of the press system, a flange end is forced into a curling die from the side of a flange open end and forming is accomplished by one step or process.
Therefore, the extreme end of the curl at which wrinkles tend to occur most easily is reduced in diameter in a free state without being supported. When the wall thickness becomes thin, wrinkles tend to occur extremely easily in that portion.
In the case of the conventional rotary rail system, a can end is inserted into a rail, and forming is accomplished while rolling the can end. Therefore, a uniform peripheral force does not act on the outer peripheral portion of an annular flange of a can end. Further, the can end moves while sliding on the curling center guide. So, when the can end material is thin, occurrence of the aforementioned problems (1) to (4) cannot be avoided.